This invention relates to a radiation image storage panel using a stimulable phosphor, and more particularly to a radiation image storage panel capable of providing an image of high sharpness and methods for preparing the same.
In the prior art, the so called radiophotography employing a silver salt for obtaining radiation images has been utilized. In recent years, particularly on account of the problem of exhaustion of silver resources on a global scale, it would be more desirable to picturize radiation images without recourse to a silver salt.
As the substitute method for the above radiophotography, there is proposed the method in which radiation transmitted through an object to be photographed is absorbed in a phosphor, and then exciting the phosphor with a certain kind of energy to permit the radiation energy stored in the phosphor to be radiated as luminescence, which luminescence is in turn detected to produce an image. More specifically, for example, U.S. Pat. No. 3,859,527 and Japanese Unexamined Patent Publication No. 12144/1980 propose a radiation image storage method in which a stimulable phosphor is used as the phosphor and electromagnetic radiation selected from visible light and IR-rays is used as the excitation energy. This storage method employs a panel having a stimulable phosphor layer formed on a support and obtains an image corresponding to the intensity of light by storing radiation energy corresponding to the intensity of the radiation transmitted through an object to be photographed in the stimulable phosphor layer of the panel and then scanning the stimulable phosphor layer with a stimulable excitation ray (hereinafter called merely as "excitation ray") thereby to take out the stored radiation energy as signals of light. The final images may be reproduced as a hard copy or reproduced on a CRT.
As is well known in the art, sharpness of the image in the radiophotography of the prior art is determined depending on spreading of momentary emission (emission on irradiation of radiation) of the phosphor in the screen. In contrast, sharpness of the image in the radiation image storage method utilizing a stimulable phosphor as described above is not determined by spreading of the stimulated emission of the phosphor in the radiation image storage panel, namely spreading of emission of the phosphor as in the case of radiophotography, but determined depending on spreading of the excitation ray within the panel. For, according to the radiation image storage method, the radiation image information stored in the radiation image storage panel is taken out sequentially, and therefore the stimulated emission by the excitation ray irradiated at a certain period of time (t.sub.i) is collected desirably wholly and recorded as the output from a certain image element (x.sub.i, y.sub.i) on the panel on which the excitation ray is irradiated; if the excitation ray is spread through scattering, etc. within the panel to excite the phosphor existing outside of the irradiated image element, an output from a wider region than the image element is recorded as the output of the above image element (x.sub.i, y.sub.i). Accordingly, provided that the stimulated emission by the excitation ray irradiated at a certain period of time (t.sub.i) is only the emission from the image element (x.sub.i, y.sub.i) on the panel on which the excitation ray is directly irradiated, sharpness of the image obtained will not be influenced by the emission regardless of the extent of spreading thereof.
The radiation image storage panel to be used in the radiation image storage method as described above has at least a phosphor layer comprising a stimulable phosphor. The phosphor layer is generally provided on an appropriate substrate. Further, as a usual practice, a protective layer for protecting physically or chemically the phosphor layer is provided on the layer surface on the side opposite to the surface to be contacted with the substrate. In such a radiation image storage panel of the prior art, the mean free path of the excitation ray within the phosphor layer will be elongated by scattering, etc. of the excitation ray, whereby the excitation ray is spread relatively greatly within the phosphor layer to be disadvantageously deteriorated markedly in sharpness, and improvement of this drawback is strongly desired.
As the methods for improvement of sharpness of the radiation image storage device, there have been known the method in which white powder is incorporated in the phosphor layer of the radiation image storage panel, as disclosed in Japanese Unexamined Patent Publication No. 146447/1980, and the method in which the radiation image storage panel is colored so that the mean reflectance in the exicited wavelength region of the stimulable phosphor is smaller than the mean reflectance in the stimulation emission wavelength region of the stimulable phosphor as disclosed in Japanese Unexamined Patent Publication No. 163500/1980. However, these methods are not satisfactory, because improvement of sharpness will necessarily result in marked lowering in sensitivity.